In a year 1935 a German scientist Karl Fischer published a method for determination of water content in samples. This was a titrimetric method based on Bunsen reaction used for determination of sulfur dioxide in aqueous solutions:
SO2 + I2 + 2 H2O → H2SO4 + 2 HI
Karl Fischer discovered that if sulfur dioxide is added in the excess the same reaction can be used for water determination by titration of the produced acids. His base of choice was pyridine which was allegedly “just standing in the rack”. In his honor the titration is named Karl Fischer titration (KF titration for short).
In the following years both original stoichiometry and reagents were revised.
Fischer presented the reaction which gave incorrect molar ratio assuming aqueous Bunsen reaction in which methanol functions only as a solvent.
The error was corrected by Smith, Bryant and Mitchell who discovered that pyridine acts only as a buffer substance, giving us the reaction which is used today:
H2O+ I2 + [RNH]+SO3CH3- + 2 RN → [RNH]+SO4CH3- + 2 [RNH]+I-
The titration was conducted manually at first. The endpoint was signaled by persistence of brown color from the added excess of iodine. This was not only slow, but also not suitable for colored samples.
Nowadays KF titration is automated and widely used for water determination in various industries.
Karl Fischer Titration Techniques
There are two different techniques for water determination by Karl Fischer: Volumetric KF titration and coulometric KF titration.
Volumetric determination is suitable for determination of water content down to 1% of water. The sample is dissolved in KF Solvent (usually methanol based) and the iodine is added as a part of a KF Reagent containing sulfur dioxide and iodine dissolved in pyridine and methanol. The endpoint is determined potentiometrically.
Figure 1. Volumetric KF titrator
Coulometric KF analysis requires only one, iodide-containing, solution. Iodine needed for KF reaction is produced by anodic oxidation of iodide from solution and the endpoint is detected electrochemically. Coulometric determination is best suited for samples with less than 1% of water.
Figure 2. Coulometric Karl Fischer titrator
In both techniques the sample is transferred to a titration vessel, dissolved and then titrated. Some samples aren’t soluble in any suitable solvent and some can cause side reactions with Karl Fischer reagents. This is when volumetric or coulometric titrator can be coupled with oven. The sample is heated in the oven and the water released from the sample is carried by a dry gas flow to a titration cell where it reacts with KF reagent. This is a perfect solution for poorly soluble samples, samples which can cause side reactions with solvent and for highly hydroscopic samples when handling with the sample in the laboratory conditions can give falsely higher results.
Karl Fischer Titration Reagents and Applications
Reagents for Karl Fischer titration are available in a broad range, depending on the purpose and ecological awareness of the user. Pyridine can be replaced with stronger base (and less smelly) imidazole, and ethanol can be used instead of methanol as a more “green” choice of solvent. Different auxiliaries are available, such as additives for fats and oils, reagents for determination of low water contents, solubilizers for poorly soluble substances, and buffer solutions for strongly alkaline or strongly acidic samples.
Karl Fischer titration is widely used for direct analysis of water content in various industries, as a reliable and robust method. In food industry it is used for water content determination in fruit juices, honey, flour, noodles, chips, cocoa powder, in petroleum industry for all kinds of different oils, gasoline, kerosene, and petroleum, in cosmetic industry for determination of water in shampoos, creams, lipstick, tooth paste, in pharmaceutic industry for raw materials, active substances, lyophilized substrates, tablets, ointments, oils. It is used for determination of water in silk, wool, wood, paper, and even in building materials such as zeolite and cement.
The list just goes on.
This is just the proof that with the right choice of technique and suitable reagents Karl Fischer titration can be suitable for water content determination in almost all imaginable samples. It is no surprise that this titration is a go to technique in any laboratory, in any industry.
Sources and Further Reading